Strong subsurface-fortified glass and glass-ceramic articles are known, being described in U.S. Pat. Nos. 3,746,526 and 3,597,305 to Giffen. Such articles are constructed of glass or glass-ceramic laminates comprising at least five distinct layers, and are formed by fusing together glass layers in a softened or molten condition to produce a laminated sheet and thereafter forming the laminated sheet while soft into an article of the desired configuration.
As in the case of three-layer laminated articles, such as described by Giffen et al in U.S. Pat. No. 3,673,049, subsurface-fortified articles comprise compressively-stressed glass surface layers fused to and substantially enveloping the core portions of the article, which compressively-stressed surface layers impart increased modulus of rupture strength thereto. In addition, however, subsurface-fortified articles comprise at least one compressively-stressed layer in the interior of the article near the surface thereof, which is intended to retard crack propagation from the surface into the interior of the article and thus reduce the incidence of breakage. Hence, even in cases where the surface compression layer of such an article is penetrated by a fracturing impact or abrasion, the subsurface compression layer can prevent the fracture from propagating to the central tensilely-stressed core portion to cause breakage, and the damaged article can continue to be used.
A preferred configuration for a subsurface-fortified glass or glass-ceramic laminate is a seven-layer configuration consisting of a tensilely-stressed central core portion, a pair of compressively-stressed subsurface fortifying layers fused to and substantially enveloping the central core portion, a pair of tensilely-stressed outer core layers fused to and substantially enveloping the core and subsurface compression layers, and a pair of compressively-stressed surface layers fused to and substantially enveloping the core, subsurface compression layers, and outer core layers. In a typical embodiment, the central core portion of the seven layer laminate is about 0.080 inches thick, each subsurface fortification layer is about 0.001 inch thick, each outer core layer is about 0.007 inch thick, and each surface is about 0.002 inch thick.
Three-layer glass laminates comprising high-expansion spontaneous opal core glasses of alkali aluminosilicate composition and low-expansion skin glasses of calcium aluminosilicate composition have been fabricated into lightweight glass plates, cups, bowls and the like according to the teachings of Giffen et al. in U.S. Pat. No. 3,673,049, to provide strong serviceable tableware. However, further improvements in the strength and durability of such laminated articles have been sought. It was anticipated that the fabrication of tableware from subsurface-fortified glass laminates might provide improvements in product durability, particularly with regard to reducing the incidence of delayed breakage, i.e., breakage occuring at some finite time after the introduction of surface flaws into the surface of the article. The subsurface fortification layer was expected to prevent breakage even in cases where flaw-induced fractures penetrated the surface compression layer and outer core of the fortified article, since breakage normally will not occur until a fracture reaches the tensilely-stressed central core portion of the article.
Initial attempts to produce subsurface-fortified laminated glassware from seven-layer laminates of conventional configuration were not successful. No improvements in serviceability over the commercial three-layer ware were obtained; rather, the resistance of the seven-layer ware to breakage at impact was reduced and the susceptibility to delayed breakage was apparently increased over the corresponding properties of the three-layer ware. These results were unexpected since the mere introduction of a subsurface fortifying layer into a three-layer laminate of otherwise conventional configuration and substantial strength should not have produced a laminated glass article of decreased strength in terms of resistance to breakage at impact.
Also, in addition to the problems of marginal strength and delayed breakage, the subsurface-fortified ware occasionally exhibited surprisingly high violence of breakage, i.e., breakage into a large number of small pieces of flying glass. This mode of breakage, termed "dicing" in the art, is not deemed a desirable mode of breakage for tableware.
Both delayed breakage and violence of breakage are understood to be largely dependent upon the tensile stresses and resulting total tensile strain energy in the core portions of the stressed laminated glass. Tensile stresses in these systems are proportional to the ratio of the total thickness of compressively-stressed layers to the total thickness of tensilely stressed layers. It was recognized that the introduction of compressively-stressed subsurface fortifying layers into commercial three-layer laminates would slightly increase tensile stresses in the core portions of the laminates; however, these increases were not deemed significant and could not account for the large increases in violence of breakage which were occasionally observed.
According to theory, reductions in tensile stress wich would reduce delayed breakage and violence of breakage could be obtained by decreasing the thickness of the compressively-stressed surface and subsurface fortifying layers in laminated articles of conventional configuration. However, reductions in surface layer thickness below about 0.002 inches are undesirable because of the lowered resistance to surface abrasion occasioned thereby, and reductions in subsurface fortifying layer thickness below about 0.0005 inches would present laminating problems and limit the effectiveness of this layer as a barrier to crack propagation.
Thus the problem presented was how to simultaneously increase the strength of the laminated system while eliminating the observed tendency toward delayed and violent breakage.